This invention relates to grinding or polishing systems. More specifically, this invention relates to systems for grinding or polishing a specimen's surface on a rotatable or movable platen.
Metallographic studies are typically conducted by properly polishing and etching the surface of a specimen so that it may be examined with a microscope. The polishing operation usually involves positioning the surface of the specimen to be polished against a grinding or polishing paper or cloth mounted on a rotating, circular platen. Normally, an abrasive and wetting/cooling agent is applied to the cloth or paper to facilitate the grinding and polishing operation.
Historically, metallographers have attached grinding papers or polishing cloths to platens in several ways. One approach was to lay the grinding paper on a "wheel" (i.e., a round, rotating platen surface) wetted with water and then slip a tight fitting ring over the periphery of the paper. In some systems, this ring mated with a beveled, raised portion beyond the edge of the paper on the platen. In other systems, the paper extended to the edge of the flat platen, and the hold-down ring covered the outer edge of the paper and part of the outer edge of the periphery of the platen perpendicular to the platen surface. Use of water enhanced adherence of the grinding paper to the platen. The combination of the effect of water and the retention ring around the paper edge usually provided adequate "adhesion" so that the paper would rotate with the wheel when the metallographic specimen was pressed against it. Initially, there might be some slippage, but this would usually stop during use.
Another prior art method utilized a polishing cloth. When using a polishing cloth, the metallographer would provide a cloth of greater diameter than that of the platen by several inches. The cloth was placed over the platen surface, and a round band of steel having a diameter slightly larger than the platen's diameter, was placed over the cloth around the platen periphery. This band could be tightened by turning a screw which connected the ends of the band. The cloth edges extending down between the band and the platen were pulled to stretch the cloth tightly over the platen surface. Then the band was tightened by turning the screw. If stretching of the cloth was inadequate, the polishing rate would be reduced. Further, the cloth could be gouged or ripped, especially if unmounted specimens were being polished.
Another, more modern approach to fasten grinding paper, polishing cloths, and other preparation surfaces to the platen, was to use a pressure-sensitive adhesive (PSA) film on the back of the material--paper, cloth, etc. The PSA backing held the grinding or polishing material affixed to the platen. The strength of the adhesive affected the performance of the polishing material. If the adhesive was too weak, the PSA backed material may come off during use. If the adhesive was too strong, the material may be difficult to remove from the platen when it was to be replaced. Nonetheless, most metallographers prefer the PSA-backed grinding and polishing materials or products, rather than the non-PSA products since they are simple to use.
Some products, such as grinding paper, have a short life; whereas polishing cloths can last for many days depending upon the number of specimens polished. To reduce costs, many labs keep a polishing cloth on a platen until the cloth is completely worn. Further, a given polishing cloth is used with only one specific abrasive. Hence, most labs need a number of platens, each one being covered by a different polishing cloth or several having the same polishing cloth for use with different abrasives.
Because platens are expensive, rather than having a large inventory of platens covered with the various grinding and polishing materials, alternate approaches have been used. One alternative was to use the same platen in combination with numerous, easily removable sheet steel discs, each disc having a distinctive pressure sensitive adhesive cloth glued to one side. Any of the steel discs could then be attached to the platen using a hold-down ring around the disc-platen periphery to keep the disc in place while permitting easy and quick replacement. This approach has a disadvantage common to all such procedures. Namely, to achieve good flatness over the entire specimen's polished surface, particularly with large specimens, the specimen must periodically be moved to the extreme edge of the grinding paper or cloth surface (particularly the former). This approach is most easily done using automatic polishing devices but is nearly impossible to do with "hand" (manual) polishing. With certain grinding and polishing machines, it is impossible to move the specimen over the edge of the grinding or polishing surface without accidentally hitting the retention ring thereby damaging the specimen.
To reduce the need for multiple platens, others have used magnetic materials to attach the grinding or polishing disc to the platen. Thus, in one such alternative system, a rubberized magnetic disc is permanently attached to a platen using an extremely strong adhesive. The steel disc (with the polishing material thereon) can then be held on the platen by the magnetic field from the magnetic layer of material adhered on the platen. However, because the magnetic material will eventually wear, diminish or degrade or the magnetic field will become reduced in strength during use, this approach requires the user to eventually purchase a new platen with a magnetic material surface layer.
If grinding with silicon carbide (SiC) paper is to be performed, thin steel discs with the paper adhered thereto can be used to magnetically attach the SiC paper to the platen. Typically, after two minutes or less of grinding, SiC paper will be worn and must be removed to allow another sheet of SiC grinding paper to be attached to the disc. Thus even though the steel disc remains attached magnetically to the platen, a system using SiC paper for grinding or polishing requires constant replacement of the SiC paper.
Another magnetic attachment system uses a steel-backed disc with the surface of the disc partially covered by hexagon-shaped areas of a resin containing iron, copper or other metal particles used for grinding with added diamond abrasive. This plate is placed on and retained by the magnetic material disc attached to the platen. This disc is used after the first grinding step (for example, 120, 180, or 240 grit SiC paper depending on the material being prepared) to replace the subsequent SiC paper steps (320, 400, and 600 grit for example) and 9 or 15 .mu.m diamond is sprayed onto the surface for grinding. After use of the hexagon-shaped resin coated steel disc, a variety of grinding and polishing cloths can be used on the specimen. The cloths are typically permanently glued onto a thin, flexible sheet of steel. The cloth covered steel disc is then held magnetically on the magnetic material-covered platen. When the cloth is worn, the cloth with its steel back is discarded. Discarding the steel plate having the attached worn cloth presents an undesirable situation from the standpoint of waste disposal. It is undesirable to discard steel in this manner because steel with a permanently attached cloth is difficult to properly recycle.
Another magnetic-disc system uses a series of wire-mesh discs with diamond abrasive firmly attached to the top surface of the wire mesh or screen. Three disc grades are available: fine grind (FG), rough polish (RP), and medium polish (MP). First, the specimens may need to be ground flat using SiC paper. Then a platen with a piece of PSA-backed magnetic, rubberized material is used in combination with a disc. Thus, a FG disc is magnetically attached to the magnetic rubberized material on the platen, and the sample is ground. This is repeated using the RP and MP discs each magnetically attached to the platen. A standard final cloth polishing step is usually required to complete the preparation.
All the described magnetic disc systems generate a magnetic field that surrounds the test sample during the grinding or polishing step. In addition, for specimens that are ferromagnetic materials, including iron, cobalt, nickel, gadolinium, and Huesler alloys, grinding and polishing produce fine particles, "swarf." This swarf may adhere to the preparation surface due to the magnetic field and degrade the surface quality of the specimens being prepared.
Thus, all of the previously described magnetic attachment systems are susceptible to magnetically attracted swarf when preparing ferromagnetic materials, that can degrade the prepared surface. In addition, the previously described magnetic attachment systems do not avoid the problems of PSA product which may detach prematurely or may be extremely difficult to remove. Thus, there is a need for a system to overcome the problems associated with a magnetic field penetrating the polishing or grinding surface and the problems associated with attaching polishing or grinding materials directly to a platen using PSA backing.